JP9071157A (hereinafter referred to as Reference 1), for example, discloses a known seat slide apparatus for a vehicle. The seat slide apparatus disclosed in Reference 1 includes a lower rail fixed to a vehicle floor and an upper rail slidably engaging with the lower rail and attached to a vehicle seat. The upper rail includes a plate-shaped first upper rail and a plate-shaped second upper rail laminated on the first upper rail. In addition, a bracket is disposed between the first and second upper rails so as to rotatably support a metallic roller rolling relative to the lower rail. The upper rail is smoothly movable in the longitudinal direction of the vehicle along the lower rail accordingly. Further, the upper rail is formed to be a minimum length for supporting the vehicle seat while the lower rail is formed to be a maximum length for being arranged at the vehicle floor. As a result, the seat slide apparatus that is adjustable and movable with a large adjustment stroke is obtained.
According to the seat slide apparatus having the aforementioned configuration, for example, looseness may occur between the lower rail and the upper rail because of an assembly error of the roller relative to either the lower rail or the upper rail, dimensional variations between the lower rail and the upper rail, and the like. Therefore, according to the seat slide apparatus disclosed in Reference 1, a resin slider slidable with an inner wall surface of the lower rail is provided at a standing surface of the upper rail formed in parallel to a side surface of the lower rail to thereby restrain the looseness of the upper rail relative to the lower rail in vertical and horizontal directions. However, because the slider is constantly in contact with the inner wall surface of the lower rail, a sliding friction resistance may be generated at the aforementioned contact portion between the slider and the lower rail in a case where the upper rail moves in the longitudinal direction along the lower rail. As a result, the smooth movement of the upper rail relative to the lower rail in the longitudinal direction may be inhibited. A roller may be provided between the upper rail and the lower rail in a manner to make contact therewith in the vertical and horizontal directions so as to eliminate the sliding friction resistance. However, in that case, the size and cost of the seat slide apparatus may increase. In addition, the roller made of metal may not absorb the possible dimensional variations between the upper rail and the lower rail. Therefore, in conclusion, the smooth movement of the upper rail relative to the lower rail in the longitudinal direction may be disturbed.
In light of the foregoing, WO2010024210A1 (hereinafter referred to as Reference 2) discloses a seat slide apparatus for a vehicle including a rolling element circulation unit. The rolling element circulation unit includes a first case and a second case in which plural rolling elements are accommodated so as to roll and circulate within the first case and the second case. The rolling element circulation unit is assembled on a cut and lift portion (an operating portion) formed at an upper rail. In a case where the upper rail slidably moves relative to a lower rail, some of the rolling elements roll and circulate by making contact with an operating surface formed at the lower rail and an operating surface formed at the operating portion of the upper rail while the other of the rolling elements roll and circulate by making contact with a non-operating surface of the operating portion. As a result, the large and smooth sliding movement of the upper rail is achieved.
According to the seat slide apparatus disclosed in Reference 2, the rolling element circulation unit exercises a rolling circulation function in a state to be mounted on the operating portion of the upper rail. That is, the rolling element circulation unit functions in a state where the rolling elements make contact with the operating surfaces of the upper rail and the lower rail. Accordingly, a rolling performance of the rolling element circulation unit may depend on an accuracy of molding and/or assembly of each of the upper rail and the lower rail. Therefore, the rolling performance of the rolling element circulation unit may decrease because of variations of the aforementioned accuracy of each of the rails.
A need thus exists for a slide apparatus for a vehicle and a rolling element circulation unit for the slide apparatus which are not susceptible to the drawback mentioned above.